Fix for iOS Cocoapods default config (#205)

Summary:
Solves #203

- [x] Sample Objc and Swift apps now have default configs(i.e non-recursive header search paths)
- [x] Fix for the cocoapods default config by adding FBMacros.h file in `Core` subspec
- [x] Updated the Spec(0.6.15) so that 0.6.15 works for default configs
- [x] Updated the doc
Pull Request resolved: https://github.com/facebook/flipper/pull/205

Reviewed By: passy

Differential Revision: D9148358

Pulled By: priteshrnandgaonkar

fbshipit-source-id: e6d32d9cde7b100573bd9a9996f0ec850b269d28

docs/getting-started.md

@@ -71,7 +71,7 @@ source 'https://github.com/facebook/Sonar.git'
 source 'https://github.com/CocoaPods/Specs'
 # Uncomment the next line to define a global platform for your project
 swift_version = "4.1"
-sonarkit_version = '0.6.14'
+sonarkit_version = '0.6.15'
 
 target 'MyApp' do